1. Field of the Invention
The present invention relates to a polishing method and a polishing apparatus which are useful for polishing and flattening a surface (surface to be polished) of a substrate, such as a semiconductor wafer.
2. Description of the Related Art
In the formation of integrated circuits on a surface of a substrate, such as a semiconductor wafer, it is a general practice to deposit an insulating film, a conductive film or a semiconductive film, etc. on the surface of the substrate, and form integrated circuit interconnects in the deposited film. For the formation of such interconnects, lithography of an integrated circuit pattern with light or electron beams is carried out. In order to form fine interconnects, it is necessary to make the width of a lithography pattern as narrow as possible, which requires a shallower focus depth. This necessitates flattening of a surface of a semiconductor wafer on which lithography is to be carried out. As a method for the flattening, polishing by a chemical mechanical polishing (CMP) apparatus is generally practiced. A multi-stage CMP process is known which comprises polishing a laminate of films formed in a surface of a substrate, such as a semiconductor wafer, in a plurality of process steps.
For example, a substrate W to be polished, as shown in FIG. 6A, is prepared by forming trenches 302 in an insulating layer 300 and forming a barrier film 304 of, e.g., SiN on a surface of the insulating layer 300, and then depositing an oxide film 306 on a surface of the barrier film 304 while filling the oxide film 306 into the trenches 302. In an exemplary multi-step CMP process, a first polishing step of the surface of the substrate W is carried out to polish the oxide film 306 partway, and then a second polishing step is carried out to polish the remaining oxide film 306 and a predetermined amount (to a target value) of the barrier film 304, as shown in FIG. 6A. In this case, the first polishing step may be carried out by using a polishing liquid (slurry) which has a high polishing rate for the oxide film 306, though low in its surface irregularities-eliminating property for the oxide film 306, and then the second polishing step may be carried out by using a polishing liquid which has a high surface irregularities-eliminating property for the oxide film 306, though low in the polishing rate for the oxide film 306, so as to increase the polishing amount of the oxide film 306 in the first polishing step and to thereby shorten the overall polishing time.
As shown in FIG. 6A, when the oxide film 306 is deposited on the insulating layer 300 in which the trenches 302 are formed, depressions are formed in those portions of the surface of the oxide film 306 which correspond to the trenches 302 provided in the insulating layer 300. In order to increase the throughput, it is desirable to polish the oxide film (uppermost-layer film) 306 as much as possible in the first polishing step using a polishing liquid having a high polishing rate for the oxide film 306 and to minimize the polishing of the oxide film 306 in the second polishing step using a polishing liquid having a low polishing rate for the oxide film 306. However, the first polishing step using a polishing liquid having a low surface irregularities-eliminating property cannot flatten the depressions in the surface of the oxide film 306, and the depressions need to be eliminated by the second polishing step. It is therefore necessary to terminate the first polishing step when the oxide film (uppermost-layer film) 306 is partly left.
On the other hand, the overall polishing time becomes longer if the second polishing step is initiated when the oxide film (uppermost-layer film) 306 remains in excess. Further, since a polishing liquid used in the second polishing step generally has low polishing ability for the lower-layer barrier film 304, the second polishing step needs to be carried out over a considerably long time. When the second polishing step is thus carried out over a long time, excessive polishing may occur in the surface of the oxide film 306 in the trenches 302, forming a depression having a depth “d”, as shown in FIG. 6B, which may cause dishing or erosion. Thus, the second polishing step should desirably be initiated with an optimal thickness of the oxide film 306 to be polished.
However, a thickness of a film to be polished, like the oxide film 306, forming an uppermost surface layer of a substrate, such as a semiconductor wafer, generally varies among substrates. In addition, the polishing rate of an uppermost-layer film can decrease, e.g., due to deterioration of a consumable member of a polishing apparatus. It has therefore been generally difficult to make a thickness of an uppermost-layer film constant at the start of the second polishing step.